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Muscles of the Thumb

  1. The movements of the thumb are performed by eight muscles.
  2. Four of the muscles are situated in the forearm and four in thenar eminence. [1]
Of the four muscles in the forearm three are extensors, one for each thumb-joint. The action, however, is not entirely isolated. One of these extensors (longus pollicis) extends the thumb tip, but if the movement be continued it acts also upon the other thumb-joint. This extensor is attached to the bone differently from the similar tendon in the other fingers. Accordingly, the impossibility of extending the nail-joint while flexing the other joints does not hold for the thumb. As in the case of all the muscles thus far considered, the muscles of the thumb show clearly the integrative action of the muscular system.
  1. The short extensor of the thumb ordinarily affects the first phalanx.
  2. As the movement increases in force the whole thumb is pulled back and the wrist muscles are innervated to prevent the movement from spreading to the wrist.
  3. The short flexor of the thumb flexes the nail joint and helps to move the entire thumb toward the little finger.
  4. The chief abductor extends the nail joint, moves the middle phalanx sideways, and abducts the entire thumb.
  5. A combination of these functions (although opposite in kind) are also found in the thumb-abductor.

Muscles of the thumb
Purpose and Function:
  1. Flexor pollicis brevis, superficial head: The Flexor pollicis brevis, superficial head is a muscle located in the thenar eminence (the fleshy mound at the base of your thumb). Its primary function is to flex the thumb at the metacarpophalangeal joint (the joint where the thumb connects to the hand), allowing you to perform actions like gripping and pinching.
  2. first palmar interosseous: The first palmar interosseous is a short, unipennate skeletal muscle in the hand's interosseous compartment. It's also known as the pollical palmar interosseous and is located on the medial side of the thumb
  3. Flexor pollicis brevis, deep head: The Flexor pollicis brevis, deep head is a small muscle located deep within the base of the thumb. Its primary function is to flex the thumb at the metacarpophalangeal joint (where the thumb meets the palm), enabling powerful gripping and pinching actions.
  4. adductor pollicis, oblique head: The adductor pollicis, oblique head muscle is a key muscle in the thenar eminence (the fleshy area at the base of the thumb). Its primary functions are to draw the thumb towards the palm (adduction) and assist in rotating the thumb inwards, enabling powerful pinching and gripping movements.
  5. adductor pollicis transverse head: The adductor pollicis transverse head is one of the two parts of the adductor pollicis muscle in the thumb. It arises from the palmar base of the third metacarpal bone and inserts into the ulnar side of the base of the proximal phalanx of the thumb. Along with the oblique head, the transverse head helps pull the thumb inwards towards the palm . This action is crucial for gripping and manipulating objects.
  6. medial sesamoid bone: The medial sesamoid bone of the thumb is embedded within the adductor pollicis muscle. Together with the lateral sesamoid bone (in the flexor pollicis brevis), they act to increase the mechanical advantage of the thumb flexor tendons, enhancing power and precision in grip and pinch movements.
  7. Proximal Phalanx: The proximal phalanx muscles of the thumb are responsible for flexing the thumb at the metacarpophalangeal (MCP) joint, allowing you to bend your thumb towards your palm. These muscles are essential for gripping objects and performing fine motor tasks.
  8. lateral sesamoid bones: The lateral sesamoid bones of the thumb are embedded within the tendons of the short thumb muscles (abductor pollicis brevis, flexor pollicis brevis). These small bones act as pulleys, improving the leverage of the thumb muscles and protecting the tendons from wear and tear.
  9. adductor pollicis brevis: The adductor pollicis brevis muscles are a group of small muscles located at the base of the thumb, within the thenar eminence. Their primary function is to adduct the thumb, which means drawing the thumb inward towards the palm for actions like gripping and pinching.


Anatomy, Purpose and Function of the Thumb Muscles

The human thumb, a critical component of hand function, is controlled by a complex arrangement of muscles. These muscles are primarily responsible for the movements of opposition, repositioning, flexion, extension, abduction, and adduction. Their anatomy, purpose, and functions are essential to understanding piano technique.
  1. Anatomy: The thumb muscles can be broadly classified into extrinsic and intrinsic groups. The extrinsic muscles originate from the forearm and insert into the thumb, whereas the intrinsic muscles have both their origins and insertions within the hand.
    • Extrinsic Muscles: These include the flexor pollicis longus (FPL), which originates from the anterior surface of the radius and interosseous membrane, and the extensor pollicis longus (EPL) and brevis (EPB), arising from the posterior surface of the ulna and interosseous membrane. They are primarily responsible for flexion and extension at the thumb joints.
    • Intrinsic Muscles: The intrinsic muscles are the thenar muscles, comprising the abductor pollicis brevis (APB), flexor pollicis brevis (FPB), opponens pollicis (OP), and the adductor pollicis (AP). These muscles originate and insert within the hand, specifically around the carpal and metacarpal bones.
The coordinated action of these muscles is vital for the dexterity and functionality of the hand. They allow for precise movements and the ability to grip and manipulate objects, essential aspects of daily activities. The intricate balance and strength of these muscles also underscore the complexity and importance of hand surgery, where maintaining or restoring these functions is a primary objective.

Origin and Insertion of Muscles

The attachment of the muscular tendon to the rest-end is called the origin of the muscle. The tendonous attachment to the moveable bone is called the insertion. Between the origin and insertion of a muscle at least one articulation must intervene. In the case of several muscles, more than one joint intervenes. The entire problem of muscular action depends upon the relationship between muscle position and joint. The question as to which end of a muscle is its origin and which is its insertion cannot always be definitely answered until the movement is seen. Muscular contraction through the principle of the opposite equality of forces will exert a pull on both bones to which the muscle is attached. The bone offering the less resistance will be the one to move. Origin and insertion may change places. The origin of a muscle will be the end closer to the trunk. [2]
Muscles of the thumb
[1]thenar eminence: The thenar eminence ("palm of the hand") and the Latin word "eminentia", meaning projection) refers to the group of muscles on the palm of the human hand at the base of the thumb.
[2]longus pollicis: a muscle that arises dorsolaterally from the middle part of the ulna, extends the second phalanx of the thumb, and abducts the hand.
[3]trunk: n. [ME. tronke < OFr. tronc < L. truncus, a stem, trunk < truncus, maimed, mutilated < IE. *tronkus < base *trenk-, to press together, crowd, whence THRONG]
the main body or stem of a nerve, blood vessel, muscle, as distinguished from its branches.

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